An Ambulance is an emergency vehicle in all Grand Theft Auto's, excepting Grand Theft Auto 2 where the Medicar was used instead.
The ambulance has appeared in every Grand Theft Auto III era game with a similar/same role in each of them. As soon as the protagonist kills enough people, an ambulance will appear and no matter how the victim was killed (besides beheading) they will come to life once the paramedics attend to them.
The ambulance can be stolen and the ambulance side-missions can be toggled on or off optionally. The aim is to pick up injured people from around the city, depending which one, and take them to the hospital within a certain time limit. While in an ambulance, the horn button can be used for the siren. When the siren is switched on, the other cars will get out of the way of the ambulance, even if it is not for an actual emergency.
The ambulance is generally the same as the GTA III era variations, but the side-mission now isn't in the game anymore. The siren can still be used to get cars out of the way and now paramedics can be dispatched to Niko's location using the mobile phone to ring 911. The ambulances appear in hospitals, when Niko kills civilians or if he rings 911. Some upsides of being in an ambulance is that the paramedics can heal you for a price and at the toll booths, Niko will just get waved through as he is in an emergency vehicle.
Related ThreadsFire/Ambulance/Police Mission - last post by @ Apr 6, 2004
Solution To Ambulance Problems - last post by @ Oct 22, 2002
Anyone Notice This On Back Of Ambulance - last post by @ Dec 5, 2004
Best place for ambulance mission - last post by @ Nov 18, 2004
Hospitals/Ambulances - last post by @ Oct 10, 2007